The purpose of this research is to formulate a molecular mechanism of the action of the sodium pump in cell membranes. The principal goal is to separate the intracellular and extracellular effects of the activating and transported cations and to obtain detailed information on the relationship between cation sites in the partial enzymatic activities and the associated transport modes in human red blood cells. Current ideas on the reaction pathway by which the cardiac glycoside-sensitive Na, K-ATPase catalyzes the exchange of Na and K are derived from experimental data from two sources, enzymatic studies in systems where all vectorial and transport properties are lost and transport studies in more intact systems, often human erythrocytes, where the precise measurement of enzymatic steps is difficult. In the present work the properties of human erythrocyte ghosts will be utilized where independent control of intracellualr and extracellular compartments is possible. Stable, modified ATP or other phosphate-containing substrates can be sealed into ghosts when, following a brief pulse of light, free ATP or substrate is released. Using this new photorelease procedure with caged-ATP or caged-Pi it is now feasible to study the cation requirements for ATP:ADP exhange and for other partial enzymatic activities and the effects of Na, K and their cogeners on enzymatic phosphorylation and dephosphorylation.